JoAnne Skelly: Black gnats in the house

If you have houseplants, it is likely you occasionally have little black gnats flying around the house. Although these tiny pests are barely visible, they become annoying as they buzz around your face. The gnats are actually short-lived flies that infest the potting soil of houseplants. They can go from egg to adult in about 17 days, depending on temperature, with larvae emerging from the eggs in four to six days. They develop more quickly with higher temperatures. There can be multiple generations in a year. Adults live approximately one week and may lay up to 300 eggs.

Fungus gnats are attracted to light. Indoors, they often fly near windows. You may also see them on walls or on the plants themselves. They fly around me at night when I am sitting at the dining room table with the lights on. Since they generally hang out near potted plants, they are probably coming from the basil plant or African violets on my table. Besides African violets, geraniums and poinsettias are prone to fungus gnats.

The good news is, adult fungus gnats don’t harm plants or bite people. Their larvae, if present in large numbers in the soil, can damage roots and stunt plant growth, particularly of seedlings, cuttings and young plants. Sometimes when a plant appears wilted, it may not be suffering from a lack of water, but from an infestation of fungus gnat larvae on its roots.

To prevent potential infestations of fungus gnats, buy pasteurized or sterilized potting soil and pest-free plants. With an existing infestation, drying the soil out is the easiest way to reduce populations, since fungus gnats thrive in moist soil. Improve the drainage by adding perlite or sand to the potting mix. Never leave water standing in the saucers of plants. You can check for and trap larvae in the soil by leaving pieces of peeled raw potato cut side down in pots. Remove and dispose of the chunks containing larvae after a few days. Yellow sticky traps are available at nurseries to trap adults. You can cut the sticky traps into smaller pieces, put them on a sticks and place in pots. If a plant is severely infested, you may want to throw it away. Insecticides for fungus gnats are not recommended for use in homes.

Enjoy the beauty of your indoor plants without the pesky buzzing of fungus gnats.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.